The present invention relates generally to gas turbine engine exhaust systems, and more particularly, to means and methods for reducing exhaust gas jet noise.
In gas turbine engines, fuel is combusted in a stream of compressed core air to accelerate a flow of gas. The accelerated gas is then used to drive one or more turbines. A first turbine typically drives a compressor for producing the high-energy gas. A second turbine may also be used to power an electrical generator, such as in an industrial gas turbine or an auxiliary power unit (APU). The second shaft may also be used to drive a turboprop or a turboshaft. The gas finally passes through of an exhaust nozzle. Upon leaving the exhaust nozzle, the gas mixes with quasi-steady ambient air surrounding the gas turbine engine thereby generating noise.
A primary source of jet engine noise is the shear region of the relatively high velocity exhausted gas stream mixing with the relatively low velocity ambient air stream. For example, in auxiliary power units, the relatively high velocity exhaust gas jet mixes with the surrounding ambient air velocity which, on ground, is approximately zero. The mixing of the different velocity streams produces a large amount of turbulence and associated noise. As is known from Lighthill's equation, sound power is proportional with the velocity gradient of the exhaust gas jet.
Efforts have been made to reduce the overall noise production of aircraft for various purposes including improving quality of life near airports. Noise produced at exhaust nozzles, including those of APUs, has long been known to be a major source of aircraft noise. Various attempts have been made to reduce exhaust gas noise, including the use of mixing tabs along the downstream perimeter of exhaust nozzles. The tabs produce smoother mixing of the high velocity exhaust gas stream and the low velocity air stream, thereby reducing shear flow noise. These systems, however, result in a pressure loss within the exhaust duct, which reduces overall engine efficiency. Therefore, it would be desirable to have an exhaust nozzle with improved noise reduction characteristics without sacrificing engine efficiency.